1. Field of the Invention
The present invention relates generally to a homopolar synchronous machine and more particularly to such a machine which has a very low inertia.
2. Description of the Prior Art
Synchronous motors have been widely used as control motors such as non-commutator type motors and step motors. Such synchronous motors should ideally have low inertia but conventionally do not. Conventional high frequency generators also have high inertia resulting in long start-up times and serious temperature increases in the rotor. Homopolar synchronous synchronous machines have, of course, been known in the form of brush-less synchronous machines which are suitable for the mentioned applications, but conventionally prior art devices of the types mentioned have more inertia than is desirable.
FIG. 1 is a partial sectional view of one embodiment of a conventional dual homopolar synchronous machine, wherein the reference numerals 1a, 1b designate stator magnetic cores; 2a, 2b designate stator windings (armature windings); 3a, 3b designate rotor magnetic cores spaced from the stator cores by the dielectric gaps A.sub.1 and A.sub.2 ; and 4 designates a permanent magnet field means which can be disposed at a back yoke C of the stator yoke 9 or which can be a ring type field winding in the space between the rotor magnetic cores 3a and 3b or between the stator magnetic cores 1a and 1b. The permanent magnet is usually used in small size devices and the field winding is usually used in the larger devices. When a field winding is used, the permanent magnet 4 is replaced by a yoke, or the rotor magnetic cores 3a, 3b are formed in one piece and the rotor is formed in one piece (separate from the drive shaft 6).
Thus, in conventional homopolar synchronous machines, the field magnetic flux is passed, as shown by the arrows, through the rotor magnetic core 3b, the rotor yoke B or the permanent magnet 4, the rotor magnetic core 3a, the stator magnetic core 1a, the stator yoke C and the stator magnetic core 1b. Accordingly, the following equation applies, even though the field magnetic flux passes through the rotor shaft 6: ##EQU1## WHEREIN, L designates the total length of the stator magnetic cores 1a, 1b and the rotor magnetic cores 3a, 3b; Sa designates the area of the space; Sb designates a sectional area of the rotor yoke, Da designates the diameter of the gap; Db designates the diameter of the rotor yoke; Ba designates the average magnetic flux density in the space; and Bb designates maximum permissable magnetic flux density at the rotor yoke. Accordingly, the relation between the diameter of the space Da and the total length L is given by the equation: ##EQU2## wherein Db/Da = KD.
The torque T is proportional to Da.sup.2 and the inertia J is proportional to D.sup.4 L and accordingly, the ratio of the inertia to the torque, that is the speed change time .tau. (also called the acceleration time constant), is given by the equation: ##EQU3## Accordingly, .tau. is determined by the diameter of the space Da, and the total length L in the axial direction is limited by equation (2), whereby the output torque T and the output capacity limit is determined by the speed change time .tau..
As stated above, conventional homopolar synchronous machines cannot be designed so as to freely and separately adjust the diameter of the gap and the length in the axial direction, whereby the acceleration response time cannot be shortened and the inertia cannot be decreased. That is, when the acceleration response time is shortened, the output cannot be increased. For example, an output of only about ten to several tens of watts has been possible for an acceleration response time of several tens to several hundreds of milliseconds.